Small Engines (Lawn Mowers, etc.): Simplicity Broadmoor losing power, blown head gasket, briggs and stratton

Question
We have a Simplicity Broadmoor bought in 2006. Has had all
routine maintenance. We recently changed the blades, oil and
filter, spark plugs and air filter. One of the spark plugs
seemed to run hotter and was very black. It seems to run
fine while doing anything BUT grass cutting. Once the blades
are engaged it looses power and wants to stall out. The
local Simplicity dealer can not find any problem and simply
replaced the spark plugs and air filter (again). They said
they have a call into Briggs and Stratton, but we have not
heard anything after 2 weeks. Having a small farm, I need my
mower. Can you help?

Answer
Generally when you find a black spark plug it is a sign of the engine running rough. In my experience on a twin cylinder if one plug is black, both will be, unless the plug is more than just black. Look closely and check the plug for oil or buildup on it, these will be signs of a possible blown head gasket or severe wear. It's also a possibility you have a valve problem. You may have another problem, it still could be in the carburetor. Sometimes these things are impossible to diagnose hands off. I hope one of these tips help.